The present invention relates to distance measuring equipment (DME) capable of accurate distance measuring with less influence of multipath signals and with less spectrum spread.
In general, as a navigational guidance system for aircraft, DME (Distance Measuring Equipment) for measuring the distance from ground equipment to an aircraft has been widely employed to obtain position information of the aircraft. In the DME, an electromagnetic wave modulated by a pulse waveform is transmitted from an (interrogator for instance, an aircraft, and a reply pulse is transmitted from a transponder for instance, a ground station after the reception of the electromagnetic pulse. An aircraft determines the distance between both stations by measuring the time period from the transmission of the pulse to the reception of the reply pulse. With regard to DME for MLS, a detailed description is provided in "Microwave Landing System Phase III" published June 1978 by the FEDERAL AVIATION ADMINISTRATION and in the article by R. J. Kelly and E. F. C. LaBerge entitled "Guidance Accuracy Considerations for the Microwave Landing System Precision DME", NAVIGATION: Journal of the Institute of Navigation, Vol. 27, No. 1, Spring 1980.
For the above-mentioned modulating pulse, in view of the international nature of the DME, the following severe international standard is provided by the ICAO (International Civil Aviation Organization) ANNEX 10 and ARINC (Aeronautical Radio INC.):
(1) The pulse width should be 3.5.+-.0.5 .mu.s. PA1 (2) The fall time should be 3 .mu.s or less. PA1 (3) Preferably the rise time should be 1.6 .mu.s. PA1 (4) With respect to a signal transmitted from a transponder (a ground equipment), the absolute value of the spectrum power within a 500 KHz band at a frequency point offset by 800 KHz from the carrier frequency should be 200 mW or less, and that at a frequency point offset by 2 MHz the spectrum power should be 2 mW or less. PA1 (5) With respect to a signal transmitted from an interrogator (an aircraft equipment), the relative value of the spectrum power within a 500 KHz band at a frequency point offset by 800 KHz from the carrier frequency (relative to that at a center frequency) should be -23 dB or less, and that at a frequency point offset by 2 MHz should be -38 dB or less.
A waveform used currently and commonly as a pulse waveform conformable to the above standard, is the Gaussian waveform. Owing to the fact that the Gaussian waveform has a nature that its power spectrum also takes a Gaussian form (that is, a function obtained by Fourier-transforming the time domain Gaussian waveform also has a Gaussian form in the frequency domain), the Gaussian waveform has an excellent advantage that its spectrum can be concentrated within a relatively narrow band as compared to other pulse waveforms. However, if it is intended to determine accurate timing (pulse position) by making use of this waveform, problems as described hereunder would arise.
In general, the above-mentioned determination of a pulse position is effected by selecting a a threshold voltage corresponding to 50% of the peak voltage and measuring the time point (timing) when a rising waveform of a received signal crosses this threshold voltage.
However, since the slope of the Gaussian waveform in the proximity of the 50% point is relatively gradual, temperature induced variation of circuit element properties would cause a variation of the threshold voltage, and as a result, a distance error would be produced because the above-referred timing (pulse position) greatly changes. Especially in DME, where the distance to be measured varies over 0--200 NM (nautical miles), and hence under the reception level is subjected to a dynamic level change of 60 dB or more, detection of the above-mentioned timing becomes more difficult and it is liable to be influenced by noise.
Moreover, in the case of the Gaussian waveform, since the above-mentioned timing detection point having a 50% peak value exists at a relatively delayed point of about 1.25 .mu.s after the rise point of the waveform, it is liable to be influenced by reflected waves in the electromagnetic wave propagation path. More particularly, all the reflected waves having a delay relative to the direct wave of 1.25 .mu.s (in terms of distance, 375 m) or less will overlap the aforementioned timing detection point and will influence the voltage at the point, and hence, in association with the aforementioned fact that the slope of the Gaussian waveform at the timing detection point is gradual, it produces a relatively large error in distance measurement.
Although the distance measurement system known in the prior art can meet the required performance for the purpose of being used for aviation route measurement despite the aforementioned shortcomings, it cannot meet the accuracy required for the all-weather landing guidance system in the future in which a high measurement accuracy is required.
In order to mitigate such short comings, recently proposals have been made as will be described in the following.
One of the proposals is the Delay and Compare (hereinafter abbreviated as DAC) which was proposed as one method for preventing deviation of a timing detection point caused by variation of the threshold voltage. According to this method, the received and detected wave is divided into two, and after one has been attenuated by a predetermined amount through an attenuator (multiplied by a factor of A: A&lt;1) and the other has been delayed by a predetermined amount (D sec.) through a delay circuit, when a difference between the attenuated waveform and the delayed waveform (the latter minus the former) is derived through a comparator, the output of the comparator crosses over zero-level steeply at the crossing point between the attenuated waveform and the delayed waveform. If this time point whent the difference waveform crosses over the zero-level is employed as a timing point, then this timing point is determined only by the shape of the pulse and is independent of the amplitude of the pulse. Accordingly, by designating the most appropriate (the steepest) detection point on the used waveform through appropriate selection of the values of the abovementioned parameters A and D depending upon that waveform, the error due to variations of the threshold voltage as described above can be vastly improved.
However, even with the DAC as explained above, the influences of the input noise as well as the reflected waves in the wave transmission path as described above would appear as variations of the input waveform itself of the DAC, and hence these influences could not be eliminated.
What was proposed under the above-mentioned circumstance was the method in which the pulse waveform was improved into a waveform that is more appropriate for distance measurement than the above-described Gaussian waveform.
Among the waveforms proposed as a waveform meeting the above-described international standard and moreover being more appropriate for distance measurement than the Gaussin waveform, is known a "cos-cos.sup.2 " waveform. This means a pulse waveform in which a cosine function is used as a waveform for a rising portion (cos (-.pi./2) to cos (0) is used) and a cosine square function is used as a waveform for a falling portion (cos.sup.2 (0) to cos.sup.2 (.pi./2) is used). By appropriately selecting the period of the cosine waveform of the rising portion and the period of the cosine square waveform of the falling portion, a waveform conformable to the above-described international standard is obtained. Since this waveform is relatively linear in the proximity of its rise point, by selecting the timing detection point of the waveform derived by the above-described DAC in the proximity of its rise point, this pulse waveform can be used as a high precision distance measurement pulse which has little error due to the influence of reflected waves in the wave propagation path.
However, since this waveform has a discontinuity at the initial rise point (the first order derivative of the waveform jumps from 0 to a finite value at the rise point of the waveform), attenuation of the spectrum at a point far from the carrier frequency is not large, and therefore, the waveform has a shortcoming that the maximum power which can be transmitted is limited in order to make the absolute value of the spectrum power at the 2 MHz point conform to the standard value as per Item (4) of the above-referred international standard conform value.
In this connection, another proposal of employing a "cos.sup.2 -cos.sup.2 " waveform was also made. In this proposed method, the rising portion of the "cos-cos.sup.2 " waveform is replaced with a cosine square function (cos.sup.2 (-.pi./2) to cos.sup.2 (0) is used). In this case also, similarly to the aforementioned "cos-cos.sup.2 " waveform, by appropriately selecting the period of the cosine square function of the rising portion and the period of the cosine square function of the falling portion, a pulse waveform conformable to the above-mentioned international standard is obtained. In the case of this waveform, although attenuation of a spectrum at a frequency far from the carrier frequency becomes large because the above-described discontinuity at the rise point does not exist, on the other hand since the steepest point on the rising slope (a point of inflection on the rising waveform) must be the 50% point to be selected as a timing detection point for DAC, the time from the initial rise point to that detection point becomes long, and so, this waveform has the shortcoming that it is liable to be very much influenced by the reflected waves in the wave transmission path as described previously.
On the other hand, a spectrum characteristic required for the waveform transmitted from the transponder (ground equipment) is, as provided in Item (4) of the above-described international standard, that the absolute values of spectrum power within 500 kHz bands at the points offset by 800 KHz and 2 MHz, respectively, from the carrier frequency should not exceed 200 mW and 2 mW, respectively. Consequently, the maximum effective radiation power (the maximum ERP) allowed to be transmitted by employing a given waveform would be determined by the degree of attenuation of this waveform at the 800 KHz and 2 MHz offset points, respectively. However, the ratio of the spectrum powers at the 800 KHz and 2 MHz offset points required for the transponder (ground equipment) is 20 dB(=10 log 200/2) as described above, and this is more severe by 5 dB at the 2 MHz offset point than the value required for an interrogator (an aircraft equipment) of 15 dB(=38-23). This is due to the fact that in contrast to the interrogator, which is equipped located on an aircraft and is largely restricted in size and weight, the transponder is located on the ground is not subjected to such restrictions, and hence it is intended to mitigate the influence of adjacent channels as much as possible in the transponder. Accordingly, if a waveform well matched with an interrogator is used, then the attenuation of the spectrum at the 2 MHz offset point is not sufficient for a transponder, and consequently, the allowable maximum ERP would be degraded by about 5 dB (about 30% in terms of power).